Technology for integrating metals and resins is needed in many different fields of industry, such as in manufacturing of parts for automobiles, consumer electrical products, industrial machinery or the like and many adhesive agents have been developed for this purpose. Some very excellent adhesives have been proposed. For example, adhesives that exhibit their function at normal temperature or with heating are used to integrally join metals and synthetic resins and this method is currently a common joining technique.
On the other hand, more rational joining methods that do not involve the use of an adhesive have been studied heretofore. An example is a method in which a high-strength engineering plastic is integrated with a light metal such as magnesium, aluminum, an alloy of these or an iron alloy such as stainless steel without the use of an adhesive. For instance, the inventors proposed a method in which a molten resin is injected onto a metal part that was inserted preliminarily into a metallic mold for injection molding, thereby forming a resin part and at the same time this molded article and the metal parts are joined (hereinafter this will be referred to as “injection joining”).
Known technology related to this injection joining is a manufacturing technique in which a polybutylene terephthalate resin (hereinafter referred to as PBT) or a polyphenylene sulfide resin (hereinafter referred to as PPS) is joined by injection joining to an aluminum alloy (see Japanese Patent Application Laid-Open No. 2004-216425: Patent Document 1, for example). A joining technique has also been disclosed in which somewhat large holes are made in an anodized film on a piece of aluminum and a synthetic resin is made to penetrate into these holes and adjoined thereto (see WO/2004-055248-A1: Patent Document 2, for example).
The principle behind this injection joining in Patent Document 1 is as follows. An aluminum alloy is immersed in a dilute aqueous solution of a water-soluble amine compound, the aluminum alloy is finely etched with a weakly basic aqueous solution and at the same time the amine compound molecules are adsorbed to the surface of the aluminum alloy. After undergoing this treatment, the aluminum alloy is inserted in a metallic mold for injection molding and a molten thermoplastic resin is injected under high pressure.
Here, the amine compound molecules adsorbed to the surface of the aluminum alloy encounter the thermoplastic resin and heat is generated. At substantially the same time as this heat generation, the thermoplastic resin is quenched by coming into contact with the aluminum alloy that is held at a mold temperature which is lower than the melting temperature of the thermoplastic resin. The resin that was apt to be crystallized and solidified here is not crystallized as quickly because of the generated heat and gets into ultrafine recesses on the aluminum alloy surface. Consequently, with the composite of aluminum alloy and thermoplastic resin, the resin is securely joined (fixed) to the aluminum alloy and is not separated from the aluminum alloy surface. That is, when an exothermic reaction occurs, a strong injection joint is produced. It has actually been confirmed that PBT or PPS, which can undergo a chemical reaction with an amine compound, can be joined by injection joining to an aluminum alloy. Another well known technique involves chemically etching the surface of a metal part preliminarily, then inserting the metal part into the mold of an injection molding machine and performing injection molding with a thermoplastic resin material (see Japanese Patent Application Laid-Open No. 2001-225352: Patent Document 3, for example).
However, although the joining principle in Patent Document 1 by the inventors does exhibit an extremely good effect with aluminum alloys or the like, it has not effect in injection joining to other metals besides aluminum alloys. Accordingly, there has been a need for the development of a novel technique for joining metals and resins. The inventors discovered such a novel technique in the course of making improvements to their method for joining a hard resin by injection joining to an aluminum alloy. Specifically, conditions were discovered under which injection joining will be possible without any chemical adsorption of the amine compound to the metal part surface or, in other words, without the help of a special exothermic reaction or any particular chemical reaction.
At least two conditions are necessary. The first condition is that a hard resin of high crystallinity be used, that is, that PPS, PBT or an aromatic polyamide be used and, furthermore, that these be suited to injection joining to obtain a further improved composition. Another condition is that the surface layer of the metal part have a suitably rough shape and that the surface be hard.
For example, when a shaped material in which a magnesium alloy serves as the material is used, corrosion resistance is low for a magnesium alloy still covered with a natural oxidized film, so a surface covered with a hard ceramic material can be obtained by subjecting this to chemical conversion treatment or electrolytic oxidation treatment and converting the surface layer into a metal oxide, a metal carbonate or a metal phosphorus oxide. Magnesium alloy parts having these surface layers come close to meet the above-mentioned conditions.
Theoretically, these shaped magnesium alloys with their surface treated are considered as follows, assuming that they are inserted into a metallic mold for injection molding. The mold and the inserted shaped magnesium alloy are generally held at a temperature lower than the melting point of the resin being injected by at least a hundred and several tens of degrees, so there is a high possibility that the temperature of the injected resin may have dropped below its melting point at the time when it is quenched upon entering the channel inside the mold and comes into contact with magnesium alloy part.
Regardless of the crystalline resin, when it is rapidly cooled to below its melting point, it does not become crystallized and solidified immediately (that is, in zero time) and there is some time, albeit extremely short, for the resin to remain in a molten state below the melting temperature or, in other words, in a super-cooled state. If the recesses in the shaped alloy are relatively large with a diameter of several hundred nanometers, then it is possible that the molten resin penetrates into these recesses within the limited time from a super-cooled state to creation of microcrystals. To put this in another way, if the numerical density of the macromolecular microcrystal group that is produced is still low, then the resin can sufficiently penetrate into the recesses as long as the recesses are large with an inside diameter of several hundred nanometers. This is because the size of the microcrystals, specifically microcrystals in which a molecular chain behaving irregularly has undergone a change into some kind of state with order in the molecular chain, is considered to be from several nanometers to 10 nm, as estimated from a molecular model.
Consequently, although the penetration of microcrystals into ultrafine recesses with a diameter of 20 to 30 nm cannot be considered a simple matter, it is concluded that the microcrystals can penetrate as long as the recesses have a diameter of about several hundred nanometers. However, since countless microcrystals are simultaneously generated, the viscosity of the resin flow rises abruptly at the distal end of the injected resin and at places in contact with the mold metal faces. Therefore, if the recesses have a diameter of about 100 nm, the resin may not be able to penetrate all the way to the bottom but will be crystallized and solidified after penetrating considerably into the interior, so fairly good joint strength (fixing strength) is produced. Here, even if the surface of the shaped magnesium alloy is an amorphous layer or a ceramic microcrystal group such as a metal oxide, the resin will be securely anchored within the recesses, provided that the surface layer is hard and strong and has a textured face on the nanometer order, hence, the solidified and crystallized resin will not readily come out of the recesses, which means that joint strength is improved. This textured face on the nanometer order presents a coarse surface as a visual image viewed with an electron micrograph.
Improving the resin composition that is injected is actually the most important element in the present invention. This relationship will be described. When the resin composition is molded for injection molding, it is quenched from a molten state to a temperature below its melting point and attempts to be crystallized and solidified, where a resin composition that is crystallized slowly can afford better joint strength. This is a requirement for resin compositions that are suitable for injection joining.
Based on this, the inventors proposed a technique in which a shaped magnesium alloy is chemically etched and then subjected to chemical conversion treatment or another such surface treatment as mentioned above to make the surface layer ceramic, which allows a hard crystalline resin to be joined by injection joining to this and high joinability to be obtained (Japanese Patent Application Laid-Open No. 2007-301972). This proves the possibility of injection joining even without the chemical adsorption of an amine compound and, when horizontal development is taken into account, also suggests that injection joining can be performed using a PBT or PPS that has been improved for injection joining, as long as at least surface configuration and surface properties are the same for all metals and metal alloys.
Let us now describe what has been disclosed as prior art. Patent Document 3 discloses a method in which chemically etched copper wire is inserted into a metallic mold for injection molding and PPS or the like is injected to produce a lead wire-equipped battery cover having a shape such that several copper wires pass through the middle portion of a PPS disk. According to the technology, even if the internal pressure of the battery rises due to the bumps (roughness) on the surface of the copper wire formed by chemical etching, gas will not leak out through the lead wire part.
The technology disclosed in Patent Document 3 is not the injection joining technology insisted by the inventors but is instead technology that is an extension of existing injection molding technology and is merely one that utilizes the difference in the linear coefficient of expansion of metals and the molding shrinkage of resins. If a resin is injected into the peripheral portion of the structure in which a metal rod-like piece passes through the resin portion, then the molded article is parted from the mold and allowed to be cooled, the rod-like piece is in a situation of being pressed by the surrounding molded resin portion. The reason is that the linear coefficient of expansion of a metal is at most 1.7 to 2.5×10−50 C−1 for an aluminum alloy, magnesium alloy, copper or copper alloy and, even if the molded article has been removed from the mold and cooled to room temperature, the shrinkage of metal will be on the level of the linear coefficient of expansion multiplied by about 100° C. and will be no more than 0.2 to 0.3%.
Further, the object of the technology is to keep gas from leaking out through the joint between the metal and the resin and the technology is premised on the fact that substantially a slight gap is formed, while it is not specifically aimed in the technology to secure the two parts. In other words, it is essential there that a labyrinth effect prevents gas from easily leaking out. Furthermore, concerning with a resin, the molding shrinkage is about 1% for PPS, 0.5% for PPS containing glass fiber and, even for a resin in which the filler content has been increased, the resin part will always shrink more than the metal part after injection molding. Therefore, if a shaped article in which the metal part is disposed in the center and this metal part goes through the resin part is produced by injection molding with an insert, an integrated product can be manufactured in which the metal part is not likely to come loose due to the pressing effect produced by molding shrinkage of the resin portion.
This method for manufacturing an integrated metal and resin product by pressing effect is known conventionally and is used to fabricate a knob on fuel oil stove as an example of a similar molded article. This method involves inserting a thick iron needle with a diameter of about 2 mm into a metallic mold for injection molding and injecting a heat resistant resin or the like into the mold. Jagged cuts (such as knurling) are formed around the needle and the resin is fixed to this so that there may be no movement. In the technology disclosed in Patent Document 3, the texturing process is changed from a physical process to a chemical process, which makes the working process simple as well as the bumps finer and improves gripping effect by using mainly a resin that is hard and crystalline for the resin part.
With the present invention there is no need at all for a pressing effect of resin. A powerful force is required to break an article in which flat plates have been joined. If the joined state of the metal and thermoplastic resin is to be maintained stably over an extended period, it is actually necessary for the linear coefficients of expansion of the two materials to be close in values. The linear coefficient of expansion of a thermoplastic resin composition can be lowered considerably by containing a large amount of glass fiber, carbon fiber or other such reinforcing fiber, where the limit to this is 2 to 3×10−50 C−1. Kinds of metals that approach this value at close to normal temperature are aluminum, magnesium, copper and silver.
The present invention is related to a technology that makes it possible for a hard resin to be joined by injection joining to a titanium alloy. The linear coefficient of expansion of a titanium alloy is about 0.9×10−50 C−1, which is lower than half value of the above-mentioned metal group. In this sense, research and development into injection joining conducted by the inventors were put aside but it seemed to be very likely that success would be possible if the temperature range for use were narrower, so research and development into titanium alloys were conducted. If it could be confirmed that injection joining was possible with titanium alloys according to the above-mentioned hypothesis of the inventors, then this would prove that the hypothesis is correct.
The specific gravity of a titanium alloy is about 4.5, which is about 60% that of iron (specific gravity of 7.9), but titanium alloys are on a par with iron and iron alloys in terms of hardness and strength and are used as high-strength, lightweight metals. Titanium alloys also resist chlorine ions, as in case of salt water or sea water, and have exceedingly good corrosion resistance in outdoor applications. Therefore, titanium alloy parts are frequently used in various mobile electronic equipment, medical instruments, automotive mounted equipments, automobile parts, marine machineries, other such parts used in movable devices and particularly in the casings and housings of equipments that may be exposed to drops of salt water or sea water. The required mechanical fixing strength and durability can be ensured and, furthermore, when a hard resin is injected onto a titanium alloy, the production of these equipment casings is considered to be extremely easy. Moreover, titanium alloys do not irritate skin or body. These alloys are therefore known to be extremely important as metals that are accepted by body and are used for prosthetic legs and hands, for example.
Let us review the conditions important for the injection joining of metals and resins by summing the hypothesis by the inventors. Specifically, to obtain good injection joining strength, it is necessary at least on the shaped metal side:
(1) that the surface have large bumps (roughness) obtained by chemical etching and that the period thereof be at least a few hundred nanometers, preferably at least 1 μm and even more preferably have a mean bump period of 1 to 10 μm;
(2) that the surface have an ultrafine textured face on the nanometer order so that it may be sufficiently hard and non-slip, that is, that the surface look coarse when viewed microscopically; and
(3) that a high-hardness, crystalline resin can be used as the resin, while preferably a resin composition that has been improved so as to delay crystallization during quenching.
This hypothesis proved to hold true not only for magnesium alloys but also for copper and copper alloys. The coarse surface mentioned in (2) above is a level that can only be observed with an electron microscope and to put this more as a general rule, high injection joining strength can be obtained when the spacing period is 10 to 500 nm and the height and/or depth is at least 10 nm.